Vibrochamp burning up 470 cathode resistor?

[waspclothes]

Hey everyone, I have a silverface Vibrochamp that was sounding really bad - I opened it up and noticed the 1W 470 cathode resistor was burnt up and this voltage was measuring over 100V where the schematic shows +21V.

I replaced it with another 1W 470 resistor and also swapped out the 25/25 cap for a new one. Within minutes, the 1W resistor was smoking and the amp was making a crackling noise - and the +21V cathode bias voltage was jumping around with the crackles.

I noticed some other vibrochamp schematics show this resistor being 2W, so I replaced it with a 7W resistor I had on hand. The resistor climbed to over 130 degrees C (250F!) and then the amp started crackling again and the bias voltage started jumping around.

My last guess was that the 6V6 had been damaged somehow by the original failed 470 cathode resistor and the resulting 100V bias voltage, so I swapped out the 6V6 for a new JJ. The amp hasn't made any crackling noises and no smoke is coming out, but my 7W 470 resistor still gets to around 100 degrees C (212F). Is this normal?

[Helmholtz]

If you get a stable 21V cathode voltage and the amp sounds good now, the problem was the tube.

The cathode resistor has to dissipate 0.94W and its temperature solely depends on dissipation power and its surface area, not on its wattage rating.
A larger sized resistor will always stay cooler than a smaller one independent of wattage rating.

[waspclothes]

Hey everyone, I have a silverface Vibrochamp that was sounding really bad - I opened it up and noticed the 1W 470 cathode resistor was burnt up and this voltage was measuring over 100V where the schematic shows +21V.

I replaced it with another 1W 470 resistor and also swapped out the 25/25 cap for a new one. Within minutes, the 1W resistor was smoking and the amp was making a crackling noise - and the +21V cathode bias voltage was jumping around with the crackles.

I noticed some other vibrochamp schematics show this resistor being 2W, so I replaced it with a 7W resistor I had on hand. The resistor climbed to over 130 degrees C (250F!) and then the amp started crackling again and the bias voltage started jumping around.

My last guess was that the 6V6 had been damaged somehow by the original failed 470 cathode resistor and the resulting 100V bias voltage, so I swapped out the 6V6 for a new JJ. The amp hasn't made any crackling noises and no smoke is coming out, but my 7W 470 resistor still gets to around 100 degrees C (212F). Is this normal?

Makes sense, thanks Helmhotlz - I think worrying about the temperature of the resistor is more a result of me recently finding a cheap FLIR camera

Regarding the tube though, are we able to say what is "wrong" with a tube doing that? My understanding of tubes is basically black-boxes at this point, I just know that it was amplifying fine until it started drawing too much current from the cathode..

[Enzo]

My last guess was that the 6V6 had been damaged somehow by the original failed 470 cathode resistor

Far more likely your 6V6 failed and that burnt out the resistor.

[Helmholtz]

Some tubes get leaky with time. Means that air gets inside and causes current to rise uncontrollably. Other possibility could be an intermittent short between electrodes.

[pdf64]

With vintage amps, it’s worth checking that the coupling cap from the preceding stage isn’t leaking any dc, thereby, in this case, raising the power tube control grid voltage (terminal 5) above 0Vdc.

[Helmholtz]

While the tube looks to have been the culprit, a 1W resistor in this place is definitely underrated. Dissipation at idle is already around 1W and will most certainly increase when the tube is driven into clipping.
Something Fender engineers might not have considered.

[eschertron]

Something Fender engineers might not have considered.

When it comes to driving the snot out of vintage amps, these are words worth framing and hanging on the wall